Migration Total mass transfer

Indirect migration assessment by compositional analysis of plastics Assessment by worst-case assumptions of total mass transfer... 

In all cases under the premise of total mass transfer where an indirect migration assessment demonstrates the impossibility of exceeding a given legal SML restriction criterion, full compliance testing has been achieved and no further migration assessment or testing is necessary. 

Migration of the reacting ion in the electric field, briefly referred to in Section II,B, is usually suppressed by the addition of excess inert electrolyte. Incorrect values for mass-transfer rates are obtained if migration contributes more than a negligible fraction of the total limiting current. 

In the bulk solution (away from the electrode), concentration gradients are generally small, and the total current is carried mainly by migration. All charged species contribute. For species j in the bulk region of a linear mass-transfer system having a cross-sectional area A, q = j or... 

Migration is the process by which ions move under the influence of an electrostatic field. It is often the primary mass-transfer process in the bulk of the solution in a cell. The electrostatic attraction (or repulsion) between a particular ionic species and the electrode becomes smaller as the total electrolyte concentration of the. solution becomes greater. It may approach zero when Ihe reactive species is only a small fraction, say l/KK), of the total concentration of ions with a given charge. 

Equation 2.11 is the basic mass transfer equation for an electrochemical system under an electric field. In Equation 2.11, the first term is the diffusion, the second term is the migration, and the third term is the advection contribution to the total mass transport of the species i in an electrochemical system. In clay soils where the hydraulic advection is negligible compared with the electroosmotic advection, the velocity term v is simplified as the electroosmotic velocity Veo-... 

The model indicates that a plane of area dA containing specie j moves in the x-direction from position 1 to position 2, and then to position 3. This motion is influenced by modes of mass transfer, such as diffusion due to a molar concentration gradient, migration due to an electrical field, natural or forced convection due to the kinematic velocity or a combination of these modes, mass transfer of species j can be quantified by the absolute value of the molar flux J or the mass flux J. Notice that J is perpendicular to the moving plane of species j and represents the absolute value of the vector molar flux J. The total flux can be defined as... 

If mass transfer is aided by convection, which is the case for most practical purposes, then can be predicted from eq. (7.96). However, both diffusion, migration and convection mass transfer influence the electrolytic deposition in electrowinning, electrorefining and electroplating. In this case, the total molar flux is predicted by eq. (4.2) and the current density by eq. (4.9). 

For every electron passed upward along the conductor, a corresponding amount of reduced species must move away from, or oxidised species move toward, the conductor. This continual migration of redox-active species must be coupled with redox reactions in order to transfer charge. If redox equipotential lines are totally static, the production of reduced species at the conductor must be accompanied by the simultaneous consumption of reduced species somewhere between bedrock and the water table. This would result in the almost instantaneous transfer of electrical current despite the much longer time required for mass transport of reduced species to the ground surface (see discussion on ion mobility, below). 

Figure 2.10 Analysis of a band of yeast proteins separated by ID gel electrophoresis of yeast total cell lysate and digested with trypsin. The proteins migrating to a band with an apparent molecular mass of 34 kDa were digested with trypsin and the extracted peptide mixture was analyzed by frontal ana-lysis-ESI-MS-MS. Six different proteins were identified from the mixture G3P1 yeast, G3P2 yeast, G3P3 yeast, IPYR yeast, RLAO yeast and BMH1 yeast. The position of the peptides for only two of the proteins is illustrated for clarity. The procedure described in Figure 2.8B-D was used to generate the gradient, and an uncoated capillary was used as the transfer line. The sample was loaded off-line. (Adapted with permission from Ref. 11).

If the concentration of nontarget species is higher than that of target species in a contaminated area, the final removal efficiency is unexpectedly decreased (Ottosen, Hansen, and Hansen, 2000 Ribeiro et 2000). One reason for this phenomenon can be theoretically explained by the concept of the transference number, as pointed out by Acar and Alshawabkeh (1993). The principle started from the question on how the current would be distributed among a mixture of species in the pore fluid. They assumed the current (/) to be the result of only ion migration in the free pore fluid, where the total current can be related to the migrational mass flux of each species via Faraday s law for equivalence of mass flux and charge flux, as follows ... 

Factors that may contribute to the total overpotential include slow charge transfer across the Helmholtz layer, slow reaction kinetics due to preceding or subsequent reaction steps, mass transport limitations (diffusion, convection, migration), and removal of the solvation sheet of water molecules (dipoles) surrounding each ion. 

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